1. Field of the Invention
The present invention relates to a feed screw device which moves a projection lens of a liquid crystal projector in parallel.
2. Description of the Related Art
Japanese Patent Application Publication Nos. 8-304739 and 10-171045 disclose liquid crystal projectors constructed such that three liquid crystal panels for red (R), green (G) and blue (B) lights where images are displayed are illuminated by a light source, and image lights passing through these liquid crystal panels for R, G, B are combined by a cross dichroic prism, and this combined image light is projected onto a screen by a projection lens.
Japanese Utility Model Application Publication No. 3-56925 discloses a liquid crystal projector having a movable part which moves the projection lens vertically in parallel, for preventing a trapezoid distortion of the projected image at the time of vertically moving on the screen. The above-described movable part comprises a feed screw (lead screw), and by rotating the feed screw, the projection lens which is supported by this feed screw through a nut member moves vertically in parallel relative to the liquid crystal projector main body.
In the case where such a feed screw is driven by a motor, it is necessary that a stopper is arranged at the drive end (stroke end), and that a slip mechanism such as a friction clutch is provided between the feed screw and the motor for softening the impact at the time when the projection lens side collides against this stopper, which is not disclosed in Japanese Utility Model Application Publication No. 3-56925. Since the inertia force of the feed screw, that is, the inertia force of the motor is absorbed by this slip mechanism, the above-described impact can be softened.
Furthermore, the impact at the drive end can also be softened by such a method where a switch which detects the drive end is provided instead of the stopper, and the motor is stopped and controlled when this switch detects the drive end.
However, in the case of the above described conventional feed screw device which cushions the inertia force of the motor by the slip mechanism, there has been such a problem that it is difficult to control the slip torque depending on the place where the slip mechanism should be used, which is found based on the magnitude of the deceleration. Furthermore, there has also been such a weak point that since the rotational torque of the feed screw is set smaller than the slip torque of the slip mechanism, it is difficult to transmit a large torque to the feed screw.
On the other hand, in the case of the above described conventional feed screw device with a switch which detects the drive end, there has also been such a weak point that it is difficult to accurately set the stroke of ON-OFF of the switch, and furthermore, it is difficult to accurately stop the projection lens at the drive end because of the inertia of the motor.
The present invention has been achieved in view of the above described circumstances, and has as its object the provision of an improved feed screw device with a simple structure, in which it is possible to transmit a torque larger than that of the feed screw device using the slip mechanism to the feed screw, and it is possible to accurately stop the movable member at the drive end, and it is possible to sufficiently soften the impact at the drive end.
In order to attain the above described object, the present invention is directed to a feed screw device, comprising: a feed screw which is driven by driving power of a motor; shaft bushes which are provided movably in an axial direction of the feed screw to rotatably support both end parts of the feed screw; and cushioning members through which the shaft bushes are supported by stationary parts.
According to an embodiment of the present invention, both end parts of the feed screw are supported through the shaft bushes and the cushioning members movably in the axial direction thereof, and therefore, an inertia force of the feed screw after the movable member moved by the feed screw has collided against the stopper is absorbed by the movement of the feed screw in the axial direction. Accordingly, the impact at the drive end can be softened.
Moreover, an embodiment of the present invention has such a structure where the feed screw is moved in the axial direction, and therefore, it is possible to transmit a torque larger than that of the conventional feed screw device using a slip mechanism to the feed screw. Furthermore, it is possible to forcibly stop the movable member at the drive end by using a stopper as mentioned above, and therefore, the movable member can accurately be stopped at the drive end.
Furthermore, according to an embodiment of the present invention, the above-described motor is stopped by a control device when the movement in the axial direction of the feed screw is detected by a detecting device. Thereby, the motor is certainly stopped.